Japanese patent publication No. 5-133803 discloses an infrared detection unit utilizing an optical lens for collecting the infrared radiation to an infrared sensor element. The infrared sensor element is enclosed in a housing with a top opening fitted with the optical lens. Recently, there has been proposed a technology for fabricating for a mold for a micro optical lens as disclosed in Japanese patent publication No. 2000-263556. As the infrared detection unit is required to be compact in size, such micro optical lens is most preferable. However, the micro-optical lens prepared in the above prior art is inherently given a uniform radius of curvature, and is not suitable for the infrared radiation unit when it requires a lens having non-uniform radius of curvature for effectively collecting the infrared radiation to the infrared sensor element. Therefore, there is a need to use a semi-conductor lens having a non-uniform radius of curvature. Even if such semiconductor lens is utilized for the infrared detection unit, however, there remains a problem that the infrared radiation passing though the boundary between the semiconductor optical lens and the top opening may be incident upon the infrared sensor element, whereby the infrared radiation originating from an area not intended by the lens causes false detection.